Prostaglandins are a group of hormone mediators derived from the metabolism of arachidonic acid via the cyclooxygenase enzymatic pathway. In the prostaglandin biosynthetic pathway, arachidonic acid is first converted to prostaglandin endoperoxide H2 (PGH2) by PGH2 synthases followed by the cell-specific isomerization or reduction of PGH2 to the active prostaglandins: PGD2, PGE2, PGF2α, prostacyclin (PGI2) and thromboxane (TxA2). Following enzymatic conversion, the major biologically active prostaglandins exert their actions locally on the cells in which they were synthesized (autocrine) and/or on nearby cells (paracrine) through specific G protein-coupled receptors (Smith, (1992) Am. J. Physiol., 263: F181-F191) to either stimulate or inhibit the production of second messengers. Prostaglandins elicit a diverse spectrum of often opposing biological effects including muscle contraction and relaxation, potentiation and inhibition of platelet aggregation, and vasodilation and vasoconstriction. Prostaglandins also exhibit both pro-inflammatory and anti-inflammatory effects. They synergize with other pro-inflammatory mediators such as leukotrienes and bradykinins, but attenuate interleukin-1 (IL-1) production and inhibit various aspects of leukocyte function (Giles, (1990) Trends Pharmacol. Sci., 11:301-304).
Prostaglandin E2 (PGE2) exhibits a broad range of actions in a number of tissues by binding to at least four EP receptor subtypes. It acts through pharmacologically distinct stimulatory (EP2) and inhibitory (EP3) receptor subtypes to stimulate and inhibit cyclic AMP (cAMP) formation, respectively (Sonnenburg, and Smith, (1988) J. Biol. Chem., 263: 6155-6160). PGE2 also stimulates calcium release and protein kinase C activity in the rabbit kidney collecting tubule, most likely by binding to the EP1 receptor subtype which is coupled to stimulation of phospholipase C (Hebert et al., (1990) Am. J. Physiol., 259: F318-F325). The EP4 receptor is an additional subtype of PGE2-sensitive receptor that was recently identified based on agonist effects and blockade by the antagonist AH 23848B (Louttit et al., (1992) The Eighth International Congress on Prostaglandins and Related Compounds, Montreal, 258; Coleman et al., (1994) Prostaglandins, 47:151-168). Other PGE2-sensitive receptors with distinct agonist pharmacology have been described (Milne et al., (1994) Br. J. Pharmacol., 111:79), but it is not clear whether they are different from the EP4 receptor.
Analogs of, PGE2 that are therapeutically useful will elicit or block only a subset of its actions by acting on a single EP receptor subtype. Because prostaglandin receptors are present in tissues in low abundance, the discovery of such analogs is facilitated by the cloning of the receptors. Assigning cloned receptors to a corresponding pharmacologically defined binding site is an iterative process. Defining novel subtypes requires selective compounds, which may only be developed once the receptor is cloned.
Three human receptors that bind PGE2 have been cloned. The EP1 (Funk et al., (1993) J. Biol. Chem., 268: 26767-26772) and EP3 (Regan et al., (1994) Br. J. Phamacol.,112:377-385) subtypes have been well characterized with subtype-selective compounds, but the pharmacology of the putative EP2 receptor (An et al., (1993) Biochem. Biophys. Res. Commun., 197:263-270; Honda et al., (1993) J. Biol. Chem., 268:7759-7762) is not entirely consistent with the pharmacology derived from tissue models of the EP2 receptor. In particular, the EP2-selective agonist butaprost, is inactive (Gardiner (1986) Br. J. Pharmacol., 87:45-56; Coleman, (1993) in Eicosanoids and Other Bioactive Lipids in Cancer, Inflammation and Radiation Injury, Nigan et al., eds., pp. 135-141). The pharmacology of this putative EP2 clone is more similar to that of the EP4 receptor, but it was named before the EP4 receptor had been described.
The deduced protein sequences of the cloned receptors indicate that all are members of the G protein-linked receptor superfamily, having seven putative membrane-spanning hydrophobic domains. The proteins share significant amino acid sequence similarity with other members of this family including the thromboxane (TP) receptor (Hirata et al., (1991) Nature 349: 617-620), rhodopsin and the adrenergic receptors.
The cloning of EP2 and/or additional EP receptors will facilitate identification of prostaglandins which can modulate specific effects elicited by this receptor. Since these effects will differ from those activated by other EP receptors, such compounds will have therapeutic utility.